In charged particle beam systems, such as electron microscopes or focused ion beam (FIB) systems, a column is typically used to focus a beam of charged particles onto the surface of a target to be imaged and/or processed. In a FIB column, an ion source (typically a liquid metal ion source, or LMIS) generates the initial beam of ions, which then passes into a “gun”, which typically focuses these ions into a roughly parallel beam entering the main body of the column. In the column, this beam may be blanked (i.e., turned on and off), deflected (moved around on the target surface), and focused onto the surface of the target. In some cases, the ion beam is used to mill (sputter) material in controlled patterns from the surface of a target—in these applications, the milling rate is roughly proportional to the beam current, thus higher beam currents are generally preferred to improve process throughputs. In other cases, the ion beam is used to image a target, where the impact of the ion beam induces emission of secondary electrons which can be detected and used to form the image—in these applications, the image resolution is roughly determined by the beam diameter. A beam having a lower beam current can typically be focused to a smaller diameter than a beam having a greater current, and the lower beam currents results in less damage to the target. Although an ideal beam would have all the ions uniformly distributed within a desired beam diameter, in actuality, the beam current distribution is more or less bell-shaped and tapers off away from the beam center. The image contrast may be reduced if the focused ion beam has extended “tails”.
Some applications require both imaging and milling. In particular, when a milled pattern needs to be precisely located with respect to a pre-existing feature on a target, it is necessary to first image the target with a lower current FIB, and then to switch to a higher current (typically larger diameter) FIB for milling. One important example of such an imaging/milling process is the preparation of thin “lamellae” (singular: “lamella”) of various types of samples, such as semiconductor devices and cryo-frozen biological samples. In the case of semiconductor device failure analysis, a particular region of interest (RoI) within an integrated circuit, usually containing a defective device to be analyzed, is exposed by FIB milling on both sides, leaving a thin slice (lamella) of material remaining which contains the defective device—these lamellae are thin enough for use in high voltage transmission electron microscopes (TEMs) or scanning transmission electron microscopes (STEMs) where atomic resolutions are, in principle, available. Because the lamellae are only tens of nanometers thick, and the defects being observed may be on the scale of nanometers, the milling to create the lamella is extremely precise.
During preparation of a lamella, it is necessary to switch between using large current, large diameter beams suitable for rapid milling and using lower current, smaller diameter beams for fine milling or imaging. This is typically done by changing a beam-defining aperture (BDA) through which the beam passes. BDAs are typically holes in a metal strip, allowing only charged particles that pass through the hole to form the beam. There are typically several BDAs, or holes, in a metal strip, and switching apertures typically entails moving the strip so that a hole of a different diameter is positioned in the path of the beam. The mechanical movement of the aperture strip is typically accurate to only a few microns, so it has been considered necessary to realign the beam after changing apertures.
Changing apertures not only necessitated realigning the beam, but also adjustments to the lenses. The optical requirements for forming a large current, large diameter beam and a lower current, smaller diameter beam are not the same. FIG. 28 shows a graph of the logarithm of the spot size of the beam 2810 on a sample as a function of the logarithm of the beam convergence angle 2812. The convergence angle is the angular spread at the target of the ions that are formed into the beam. The aperture in the beam path determines the beam convergence angle—a larger aperture accepts charged particles from a wider angle.
The aperture size affects several properties of the beam. A larger aperture passes charged particles that are further away from the optical axis, thereby increasing spherical aberration (proportional to the cube of the convergence angle) in the beam and, as shown in line 2802, increasing the beam spot size, that is, the beam diameter at the sample. A larger aperture also increases the chromatic aberration (proportional to the convergence angle) as shown by line 2804.
Many charged particle systems form a spot on the sample by forming a demagnified image of the source and the spot size decreases with decreasing magnification (increasing demagnification) as shown by line 2806. The magnification of the optical system decreases with increasing convergence angle. Thus, increasing the convergence angle affects the aberrations and the source demagnification, the aberration tending to increase the spot size with increasing convergence and the source demagnification tending to decrease the spot size with increasing convergence angle.
FIG. 28 shows the combined effects 2808 of spherical aberration, chromatic aberration, and source demagnification on the spot size as the convergence angle changes. At the beam settings for the graph in FIG. 28, the smallest spot size A at the low point of curve 2808 is produced at beam convergence angle B. FIG. 28 shows that if a larger or smaller aperture were used, increasing or decreasing the convergence angle, the spot size would increase, moving away from the low spot in the curve 2808. To return to a more ideal spot size, FIB users would change the lens voltages when changing apertures, to change the magnification and return the beam spot size to a new low point in a modified curve.
Thus, in the prior art, switching between a high current mode and a low current mode has required not only switching of beam-defining apertures, but also realigning the beam and changing numerous lens voltages in order to separately optimize the column ion optics for each of these two modes. Unfortunately, since some column voltages may need to change by substantial amounts (>100 V), long power supply settling times may be required when changing between modes before a column is ready to function, e.g., when switching back-and-forth between milling and imaging. Thus it would be desirable to reduce the switching times between milling and imaging modes in order to improve throughput in the preparation of thin lamellae.